1. Technical Field
The present invention relates to a method for generating a carrier residual signal and its device. More particularly, the present invention relates to a method for generating a carrier residual signal and its device, which is capable of obtaining a heterodyne optical signal used in a photometric field or an optical fiber radio communication field.
2. Related Art
In an optical communication field or a photometric field, a heterodyne method has been used in which two light waves having frequencies slightly different from each other overlap each other so as to generate a ‘beat’, and necessary information is extracted from the ‘beat’.
In recent years, as an information amount is increased by a moving picture distribution service and information contents are diversified, frequency resources of a wide band can be utilized. As a result, a wireless system has been examined in which electric waves of a millimeter wave band (30 to 300 GHz) are used. In particular, since the transmission distance is short in the millimeter wave, as disclosed in Patent Document 1, an optical fiber radio communication system has been adopted in which optical communication by using an optical fiber is used in a long distance transmission portion, and an optical communication signal is converted into a radio communication signal to be used in the vicinity of a user using radio communication or a receiver. Further, it is very difficult to generate the millimeter wave by using an electrical oscillator. However, by using a heterodyne method, an optical signal having a different frequency is inputted to an optoelectric converter (O/E converter), and an outputted electric signal is amplified, which enables the millimeter wave to be easily generated.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-353897
In order to generate two light waves having frequencies slightly different from each other, which are used in the heterodyne method, a Zeeman laser or a method of converting one light wave by using a frequency shifter is generally used. However, since the Zeeman laser uses a He—Ne laser, a device has a large size. When the frequency shifter is used, since a plurality of optical components are combined, a light source circuit becomes complicated, and characteristics may vary due to environment variation, such as temperature variation.
Further, even when a plurality of semiconductor lasers are combined, it is required for two light waves to be adjusted on the same optical axis, and output characteristics of the semiconductor lasers vary due to the temperature variation. As a result, it may not be possible to maintain the frequency difference between the two light waves to a predetermined value.
Meanwhile, as a method of easily obtaining light waves of different frequencies, the applicants have suggested an SSB (Single Side-Band) optical modulator.
An example of the SSB optical modulator is disclosed in below Non Patent document 1.
Non Patent Document 1: “X-cut LiNbO3 Optical SSB-SC Modulator” in pages 17 to 21 in “The Sumitomo Osaka Cement•technical report (2002)”, published by The institute of a new technology in Sumitomo Osaka Cement Co., Ltd. in Dec. 8, 2001.
An operation principle of an SSB optical modulator will be described.
FIG. 1 is a diagram illustrating a principle of an SSB optical modulator in which carrier components are not suppressed.
According to a structure of an optical modulator, Ti or the like is dispersed on a substrate having an electrooptic effect, such as LiNbO3, so as to form a Mach-Zehnder optical waveguide shown in FIG. 1. The SSB optical modulator is not limited to the single Mach-Zehnder optical waveguide shown in FIG. 1, but as shown in FIG. 2, an optical waveguide having a nesting MZ structure in which two sub MZ (Mach-Zehnder) optical waveguides MZA and MZB are disposed in parallel in the arms of a main MZ optical waveguide MZC may be used according to each purpose.
FIGS. 1 and 2 show a simplified structure of an electrode for applying a modulation signal or a direct current bias signal to branch waveguides of a Mach-Zehnder-type optical waveguide. RFA and RFB show simplified structures of traveling wave coplanar electrodes for applying a microwave modulation signal to two branch waveguides of a single Mach-Zehnder-type optical waveguide or the sub MZ optical waveguides MZA and MZB shown in FIG. 2. Further, DCA and DCB are simplified phase adjusting electrodes for applying a direct current bias voltage that gives a predetermined phase difference to specific branch waveguide of the single Mach-Zehnder-type optical waveguide, or the sub MZ optical waveguides MZA and MZB, and DCC is a simplified phase adjusting electrode for applying a direct current bias voltage that gives a predetermined phase difference to the main MZ optical waveguide MZC.
In the SSB optical modulation technology, it is known that an SSB modulation signal is obtained by summing the original signal and the original signal converted by using Hilbert conversion.
In order to perform the optical SSB modulation in which the carrier components are not suppressed, a dual driven single MZ modulator shown in FIG. 1 (example where a Z cut substrate is used) may be used.
While representing incident light as exp(jωt), a single frequency RF signal φ cos Ωt is inputted from a RFA port, and at the same time, a signal obtained by subjecting the signal to Hilbert conversion, that is, H[φ cos Ωt]=φ sin Ωt is inputted from a RFB port.
Since the condition sin Ωt=cos(Ωt−π/2) is satisfied, two signals can be simultaneously inputted by using a phase shifter for a microwave. In this case, φ denotes a modulated degree, and ω and Ω denote frequencies of a light wave and a microwave (RF), respectively.
Further, by applying a bias from a DCA port, the phase difference π/2 is given to the light waves transmitting in the arms of the MZ optical waveguide.
Therefore, focusing on a phase term of a light wave at the multiplexed location, it can be represented by Equation 1.exp(jωt)*{exp(jφ cos Ωt)+exp(jφ sin Ωt)*exp(jπ/2)}=2*exp(jωt)*{J0(φ)+j*J1(φ)exp(jΩt)}  (1)
In this case, J0 and J1 denote zero-order and primary Bessel functions, and the components after the primary component are ignored.
As represented by Equation 1, the zero-order and primary components remain, but a −1st (J−1) component is lost (if showing it schematically, a light wave having spectrum distribution shown at the right side of the MZ optical waveguide of FIG. 1 is emitted from the MZ optical waveguide). In addition, the frequency of the zero-order spectrum light denoted by J0 is ω, similar to incident light. The frequency of the primary spectrum light denoted by J1 is ω+Ω, and it may become a frequency that has shifted by the frequency of the microwave from the frequency of the incident light.
Further, in order to make the −1st component (J−1) remain and make the primary component (J1) removed, a bias giving the phase difference −π/2 can be applied to the DCA port. In this case, the −1st spectrum light has the frequency of ω−Ω.
Next, a method of suppressing a zero-order Bessel function which is the carrier component will be described.
FIG. 2 is a schematic view illustrating an optical waveguide of a single side-band with suppressed carrier (SSB-SC) optical modulator. In the case of the SSB-SC optical modulator, as shown in FIG. 2, it has a structure in which sub MZ interference systems are respectively provided to both arms of a single MZ interference system.
The signal shown in FIG. 3 is applied to the sub MZ optical waveguide. This can be considered to be the same situation as a case in which common intensity modulation is performed by bottom drive.
At this time, considering a phase term of emitted light, it can be represented by Equation 2.exp(jωt)*{exp(jφ sin Ωt)+exp(jφ sin Ωt)*exp(jπ)}=2*exp(jωt)*{J−1(φ)exp(jΩt)+J1(φ)exp(jωt)}  (2)
Referring to Equation 2, it can be understood that a spectrum component of an even-number order including a carrier component is cancelled (if schematically illustrating it, a light wave having the spectrum distribution shown in a right side of the MZ optical waveguide of FIG. 3 is emitted from the MZ optical waveguide).
The modulation method (SSB optical modulation) shown in FIG. 1 and Equation 1 and the modulation method (method of suppressing carrier components at the sub MZ) shown in FIG. 3 and Equation 2 are combined, so that one of the primary spectrum (J1 term) and −1st spectrum (J−1 term) may be selectively generated.
As such, as shown in FIGS. 1 and 2, modulation signals and direct current bias signals that are applied to various SSB optical modulators are appropriately adjusted, so that spectrum light having any frequency component may be outputted.
An advantage of some aspects of the present invention is that it provides a method for generating a carrier residual signal and its device, in which a heterodyne optical signal used in a photometric field or an optical fiber radio communication field can be stably generated with a simplified structure.